The 7 Secrets To Building Your Sustainable Home That No One Tells You – Part 4: The Importance Of Thermal Mass

Although the term ‘thermal mass’ is not commonly used, there are many examples where we experience it and appreciate its benefits.

The most impressive is the ocean: in winter, when there is less sunshine and the average air temperature is low, the water is chilly and only the tough ones might enjoy a swim! In spring, the sun will slowly heat up the water so that finally in summer it will have a comfortable warm temperature. Water has a great capacity of storing heat – it will stay constantly warm during day and night, and even in winter, it can be significantly warmer than its surrounding air temperature due to its ability to absorb solar energy.

Water demonstrates the principle of thermal mass.

How does it apply to construction?

Thermal mass is the capacity of an object to store heat. It is an effective way to improve thermal comfort in a building, since it will absorb heat when the surroundings are hotter than the mass, and give heat back when the surroundings are cooler.

When situated well and in combination with passive solar design, Thermal mass needs to be situated correctly and needs to work in combination with passive solar design and good performing insulation, otherwise it can have negative effects and even increase the need for heating and cooling.

Thermal mass should be situated on the interior face of the building envelope and must be thermally separated from the outside via insulative materials.

Therefore a standard brick veneer home does NOT have thermal mass.

Split systems and ducted heating are the most common heating systems in Australia – they function by pumping hot air into the room. When fan or ducted heaters are turned on, a room will be warm, however, immediately after they are switched off, it is cold again. This is because they use convective heat which warms the air, not the materials in the room, leading to thermal discomfort.

Open fire, gas or hydronic heaters eject radiant heat from their hot surfaces. It takes longer to warm a room as it also warms up the objects and materials, the occupants in turn feel more comfortable, as the Mean Radiant Temperature is well balanced. Even when the heaters are turned off, the thermal mass will release its stored heat slowly and therefore keep the room warm for a longer time, depending on the performance of the insulation. Thermal mass can play an essential role in saving energy and be used actively for heating and cooling.